Aligning the Upstream DMT Symbols of Multiple Lines in a TDD DSL System

ABSTRACT

Time-division duplex (TDD) transmission in a digital subscriber line (DSL) communication system including a group of DSL transceivers at a remote side (FTU-Rs) and a distribution point unit (DPU) by performing, with a second FTU-R among the group of FTU-Rs, an initial upstream symbol alignment for a first FTU-R, where a first gap time the first FTU-R needs to wait before transmitting first upstream symbols to a first DSL transceiver at an operator side (FTU-O) at the DPU in a TDD frame period is acquired to make the first upstream symbols align at the first FTU-O&#39;s interface, transmitting second upstream symbols to the first FTU-O to correct the first gap time after performing the initial upstream symbol alignment, receiving, from the first FTU-O, tuning offset information representing a correction for the first gap time, and tuning the first FTU-R&#39;s upstream symbols transmit time according to the tuning offset information received.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of U.S.Non-Provisional application Ser. No. 13/875,447 filed on May 2, 2013,entitled “Aligning the Upstream DMT Symbols of Multiple Lines in a TDDDSL System,” which claims benefit of U.S. Provisional Patent ApplicationNo. 61/641,424 filed May 2, 2012, by Cao Shi, et al. and entitled “AMethod to Align the Upstream DMT Symbols of Multiple Lines in a TDD DSLSystem” and U.S. Provisional Patent Application No. 61/772,312 filedMar. 4, 2013, by Cao Shi, et al., and entitled “A Method to Align theUpstream DMT Symbols of Multiple Lines in a TDD DSL System—2,” which areincorporated herein by reference as if reproduced in their entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

REFERENCE TO A MICROFICHE APPENDIX

Not applicable.

BACKGROUND

Digital subscriber line (DSL) technologies are utilized to supply highspeed data over twisted pair conductors. Current DSL standards maycomprise asymmetric DSL (ADSL, ADSL2 and ADSL2+), very-high-bit-rate DSL(VDSL and VDSL2), and integrated services digital network (ISDN). Thesetechnologies may use baseband transmission in conjunction with plain oldtelephone service (POTS). DSL signals may occupy higher frequency bandswhile the POTS signals may be transmitted over frequency bands below 4kilohertz (KHz). The DSL and POTS signals may be split and coupledthrough a splitter to the corresponding receiver and network,respectively.

Discrete multi-tone modulation (DMT) may be implemented in DSL systems.DSL access multiplexer (DSLAM) equipment may offer multi-port access andsupport to different DSL technologies. ADSL/ADSL2 and VDSL2 standardsmay employ frequency division duplexing (FDD), in which downstream (DS)and upstream (US) transmission occurs simultaneously at two differentfrequency bands. However, these standards may suffer from issues withnear-end crosstalk (NEXT) and echo during transmission.

Alternatively, time-division duplex (TDD) systems may be utilized, inwhich US and DS transmissions may occur in different time intervals. DSsignals of different lines may be transmitted at the same time since allthe transceiver units are all located and controlled at the centraloffice side. However, US signals may be transmitted at varying times andshould be aligned properly to facilitate crosstalk elimination atcentral office or operator-side transceivers. Timing offsets betweenvarious signals arriving from the customer premises may detrimentallyaffect crosstalk cancellation. Thus, there is a need to improve thealignment of US symbols as seen at operator-side equipment in order toenhance crosstalk cancellation.

SUMMARY

In one embodiment, the disclosure includes a method for time-divisionduplex (TDD) transmission in a digital subscriber line (DSL)communication system including a group of DSL transceivers at a remoteside (FTU-Rs) and a distribution point unit (DPU) including performing,with a second FTU-R among the group of FTU-Rs, an initial upstreamsymbol alignment for a first FTU-R where a first gap time that the firstFTU-R needs to wait before transmitting first upstream symbols to afirst DSL transceiver at an operator side (FTU-O) at the DPU in a TDDframe period is acquired to make the first upstream symbols align at thefirst FTU-O's interface, transmitting second upstream symbols to thefirst FTU-O to correct the first gap time after performing the initialupstream symbol alignment, receiving, from the first FTU-O, tuningoffset information representing a correction for the first gap time, andtuning the first FTU-R's upstream symbols transmit time according to thetuning offset information received.

In another embodiment, the disclosure includes apparatus for atime-division duplex (TDD) transmission in a digital subscriber line(DSL) transceiver at remote side (FTU-R) having a memory storagecomprising instructions and one or more processors coupled to the memorythat execute the instructions to enable the FTU-R to perform an initialupstream symbol alignment with a second FTU-R that supports TDDtransmission with a first gap time that the FTU-R needs to wait beforetransmitting first upstream symbols to a first DSL transceiver at anoperator side (FTU-O) in a TDD frame period acquired to make the firstupstream symbols align at the first FTU-O's interface, transmit secondupstream symbols to the first FTU-O to correct the first gap time afterperforming the initial upstream symbol alignment, receive, from thefirst FTU-O, tuning offset information that represents a correction forthe first gap time, and tune the FTU-R's upstream symbols transmit timeaccording to the tuning offset information.

In yet another embodiment, the disclosure includes distribution pointunit (DPU) including a plurality of digital subscriber line (DSL)transceivers that support time-division duplex (TDD) transmissions withrespective DSL transceivers at remote side (FTU-Rs), and

a processor coupled to the DSL transceivers and configured to perform aninitial upstream symbol alignment for a first FTU-R coupled to a firstDSL transceiver among the plurality of DSL transceivers with a secondFTU-R coupled to a second DSL transceiver among the plurality of DSLtransceivers, wherein a first gap time that the first FTU-R needs towait before transmitting first upstream symbols to the first DSLtransceiver in a TDD frame period is acquired to make the first upstreamsymbols align at the first DSL transceiver's interface, estimate acorrection for the first gap time to be used by the first FTU-R to tuneupstream symbols transmit time in response to reception of secondupstream symbols for correcting the first gap time from the first FTU-R,and transmit, to the first FTU-R, tuning offset information thatrepresents a correction for the first gap time.

These and other features will be more clearly understood from thefollowing detailed description taken in conjunction with theaccompanying drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of this disclosure, reference is nowmade to the following brief description, taken in connection with theaccompanying drawings and detailed description, wherein like referencenumerals represent like parts.

FIG. 1 is a schematic diagram of an embodiment of a system level DSLarchitecture.

FIG. 2 illustrates a schematic of two xDSL systems 200 with twoscenarios of crosstalk occurring.

FIG. 3 illustrates an embodiment of operations performed by atransmitter for construction of consecutive DMT symbols.

FIG. 4 illustrates an embodiment of signaling experienced by atransmitter (TX) and a receiver (RX) in a FDD modem.

FIG. 5 is an embodiment of overlap between a received signal and echosignals.

FIG. 6 illustrates an embodiment of loop lengths for various ports.

FIGS. 7A, 7B, and 7C illustrate embodiments of downstream symbolalignment at different time points.

FIGS. 8A, 8B, and 8C illustrate TDD upstream symbol alignment atdifferent time points.

FIG. 9 shows an embodiment of symbol alignment for ports of varying looplengths.

FIG. 10 illustrates loop attenuation versus loop length for differentloop types.

FIG. 11 illustrates an embodiment of consecutive DMT symbols for theG.fast standard.

FIG. 12 shows an embodiment of a TDD frame structure for G.fast.

FIG. 13 is an embodiment of an operator side apparatus which mayimplement timing alignment.

FIG. 14 is a flowchart of an embodiment of an upstream symbol alignmentmethod.

FIG. 15 is a schematic diagram of an embodiment of a DPU.

DETAILED DESCRIPTION

It should be understood at the outset that, although an illustrativeimplementation of one or more embodiments are provided below, thedisclosed systems and/or methods may be implemented using any number oftechniques, whether currently known or in existence. The disclosureshould in no way be limited to the illustrative implementations,drawings, and techniques illustrated below, including the exemplarydesigns and implementations illustrated and described herein, but may bemodified within the scope of the appended claims along with their fullscope of equivalents.

Depending on the supported standard, a DSL system may be denoted as anxDSL system where ‘x’ may indicate any DSL standard. For instance, ‘x’may stand for ‘A’ in ADSL2 or ADSL2+ systems, ‘V’ in VDSL or VDSL2systems, or ‘F’ in G.fast systems. When a transceiver is located at anoperator end of the DSL system, including a central office (CO), DSLaccess multiplexer (DSLAM), cabinet, or a distribution point unit (DPU),the transceiver may be referred to as an xTU-O. On the other hand, whena transceiver is located at a remote or user end such as a customerpremise equipment (CPE), the transceiver may be referred to as an xTU-R.For example, if the DSL system is a G.fast system, a transceiver at anoperator side may be referred to as a G.fast transceiver unit at anoperator side (FTU-O). Similarly, in the G.fast system, a CPEtransceiver may be referred to as a FTU at a remote terminal (FTU-R),i.e., at a subscriber side. G.fast is a most recently started ITU-TSG15-Q4 DSL standard and is in progress.

For convenience, the application is written primarily using notationfrom G.fast, but as a person of ordinary skill in the art willrecognize, the techniques disclosed herein apply to any TDD DSL system.For example, while an operator side transceiver may be labeled as FTU-Oherein to refer to G.fast, the operator side transceiver in manyinstances may be considered as an xTU-O. This principle may also beapplicable to transceivers on the customer side, in which a G.fastspecific transceiver, FTU-R, may also be referred to as xTU-R for anyTDD DSL system.

FIG. 1 is a schematic diagram of an embodiment of a system level DSLarchitecture 100. The architecture 100 may comprise an operator sidemodem or DSLAM 150 and a customer side modem customer premises equipment(CPE) 120. The CPE modem 120 may comprise a customer side transceiver121 and splitter 122. The customer equipment may include connections toa computer 110 and telephone service POTS 130. The DSLAM 150 maycomprise an office side transceiver 152 and a splitter 151. The DSLAM150 may additionally be connected to a network management system (NMS)170 and a public switched telephone network (PSTN) 160. The customerside equipment 120 and DSLAM 150 may be connected by a twisted pair line140 for data transmission.

In the US direction, a customer side transceiver 121 may receive datafrom a computer 110 and modulate the data into a DSL signal. The DSLsignal may then be sent to a splitter 122, which may integrate thesignal from the customer side transceiver 121 and the signal from POTS130. The combined signal may then go through a twisted pair line 140 tothe DSLAM 150 equipment. After processing at the transceiver 152, someof the information may be sent to a NMS 170 for monitoring purposes. Forthe DS direction, the signal may flow from the central office to thecustomer side as opposite from that of the upstream.

An xDSL technology in an architecture embodiment such as in FIG. 1 mayutilize frequencies higher than 4 KHz. Problems caused by crosstalk maybecome increasingly important as the frequency is increased. Crosstalkmay refer to interference between twisted pairs during transmission.This interference may be divided into NEXT and far end crosstalk (FEXT).FIG. 2 shows a schematic of two xDSL systems 200 with two scenarios ofcrosstalk occurring. The systems 200 may comprise an occurrence of NEXT210 and an occurrence of FEXT 220. NEXT may be defined as wheninterference occurs at the receiver of the same end of the cable fromwhich the signal was transmitted. FEXT may occur when the interferencepropagates down and occurs at the opposite end of the cable.

DSL systems such as ADSL/ADSL2/ADSL2+ and VDSL2 may utilize FDD for DSand US transmission. In systems that employ FDD, signal transmission inthe time domain may be continuous with no disruption, while there may belittle to no overlap in the frequency domain between the DS and USsignals. A timing advance technique may be used to make the echoedtransmitted signal orthogonal to the received signal in DS and UStransmission. Additionally, cyclic extensions (CE), such as cyclicprefix (CP) and cyclic suffix (CS), may be added to the symbols in orderto account for different loop lengths between the multiple lines. Thistechnique may also enable synchronization of transmission and alignmentof symbols in time among multiple lines. Synchronization may make NEXTorthogonal to the received signal in both DS and US directions. NEXT maybe filtered out and may not affect link performance; however, FEXT maystill cause the data rate to decrease and produce instability in thelink carrying service.

The VDSL2 standard may utilize DMT transmit symbols, as described inRecommendation ITU-T G. 993.2, entitled “Very high speed digitalsubscriber line transceivers (VDSL2)”, dated December 2011, which isincorporated herein by reference as if reproduced in its entirety. Thetransmit DMT symbol may be time domain samples produced from the DMTmodulator. Inverse discrete Fourier transform (IDFT) modulation of the Nsub-carriers may construct 2N real values x_(n), where n=0, 1, . . . ,2N−1, which is subsequently followed by cyclic extension, windowing, andoverlap operations.

FIG. 3 illustrates an embodiment of operations performed by atransmitter for construction of consecutive DMT symbols. The DMT symbolmay comprise 2N samples, a cyclic prefix of length L_(CP), and a cyclicsuffix of length L_(CS). The cyclic suffixes of consecutive symbols mayoverlap by β windowed samples. L_(CP) may represent the length of thelast samples of the IDFT output x_(n) attached as a prefix to the 2Noutput IDFT samples x_(n). L_(CS) may represent the length of the firstsamples of x_(n) added as a suffix to the x_(n)+L_(CP) samples. β may bethe length of the windowing function, where the first β samples of theCP of current DMT symbol and last β samples of the CS of previous DMTsymbol are employed in windowing of the transmitted signal. The windowsample values may be determined according to a vendor's discretion. Inan embodiment, the maximum value of β may be min(N/16, 255).Furthermore, the β samples (e.g., the windowed parts) of consecutivesymbols may overlap and be added to one another.

The cyclic extension (CE) length may be defined asL_(CE)=L_(CP)+L_(CS)−β, wherein values of the components may be setaccordingly to satisfy the equation L_(CE)=L_(CP)+L_(CS)−β=m×N/32, wherem may be an integer value between 2 and 16, inclusive. Support forchoosing the value of m=5 may be mandatory. In all cases, β<L_(CP) andβ<L_(CS) should hold true. CS and CP may also be partitioned accordingto a vendor's discretion. The specific settings of the CE and CP may beexchanged during initialization.

Utilizing a CS during VDSL2 transmission may be beneficial in helpingreduce spectral leakage with a transmit windowing technique. Thewindowing function may shape the envelope of the transmitted signal inorder to lower leakage issues. A CS may also make an echo signal andreceiver (Rx) signal to be orthogonal to each other due to a timingadvance technique. The timing advance may enable alignment andsynchronization of symbols at transceivers of both the office side andcustomer side.

A VDSL2 system may employ different frequency bands for DS and USsignals. However, signals may be transmitted at the same time, and VDSL2transceiver units (VTUs) may show the transmitted data as resulting echosignals. In order to attenuate an echo, a hybrid may be used. In anembodiment, a hybrid may reduce an echo by about 18 decibels (dB). FIG.4 illustrates an embodiment of signaling experienced by a transmitter(TX) and a receiver (RX) in a FDD modem 400. The modem 400 comprises thetransmitter 402, the receiver 404, and a hybrid or hybrid circuit 406.The hybrid 406 may be a circuit used to couple signals from thetransmitter 402 and the receiver 404 into one line. FIG. 4 shows thatthe signal from the transmitter 402 may leak into the signal received byreceiver 404. This leakage may subsequently show up as an echo signal.The signal received by the RX 404 is a superposition 408 of the echosignal and a desired signal as illustrated.

FIG. 5 is an embodiment of overlap 500 between a received signal andecho signals. When DS and US signals are transmitted at the same time, areceiver may obtain both the received signal from the remote side andthe echo signal from the local side. If the echo signal and receivedsignal are not orthogonal, then the echo signal may contribute tospectral leakage into the received signal band, which may further resultin the received signal's signal-to-noise ratio (SNR) degradation. A CSmay be introduced in order to keep the echo signal and the receivedsignal orthogonal. Additionally, a timing advance may be used tominimize the length of the CS.

By way of further example, suppose a signal from a VDSL2 transceiverunit on the remote side (VTU-R) is transmitted Δt behind a signal from aVDSL2 transceiver unit on the office side (VTU-O). The signal may have apropagation delay of t₀. Thus, the misalignment of the receiving signaland echo at VTU-O may be |Δt+t₀|, while the misalignment at VTU-R may be|Δt−t₀|. The value of CS may then need to be the maximum of |Δt+t₀| and|Δt−t₀|. If both VTU-O and VTU-R start at the same time, then Δt isequal to zero, and CS may have the minimum length. The timing advancetechnique in VDSL2 G.993.2 may be utilized to allow the transmittingsignals for VTU-O and VTU-R to start at the same time. In this case, amaximum CS/2 may be available in both directions for the far-end signalto arrive at the receiver without causing spectral leakage.

The ITU-T standard G.fast may provide broadband access over copper pairsfrom the fiber to the distribution point (FTTDP) near the CPE. G.fastwas developed to standardize FTTDP application scenario to addressultra-high speed access on short drop-wire copper lines fromdistribution points. G.fast may utilize TDD as the physical (PHY) layerduplexing method and DMT as modulation. In order to avoid NEXT, all ofthe ports at the same FTTDP node may need to align their DS and US timeslots. Alignment of DS symbols of all transmitters and US symbols of allreceivers at FTU-O's may be desirable for FEXT cancellation of DS and USsignals, respectively.

DS symbol alignment for multiple ports may be achieved in astraightforward manner due to the collocated nature of the FTU-O's;however the alignment of US symbols from all of the ports arriving atthe FTU-O's may not be as straightforward due to unequal loop lengths.FIG. 6 illustrates an embodiment of received symbols for various looplengths for various ports. A DMT symbol may comprise a CP 610, 2Nsamples 620, and a CS 630. The differing lengths of the three lines mayresult in timing offsets between various signals arriving from thecustomer premises. For example, US symbols from port 1, which has a looplength of 200 m, may reach an FTU-O port later than US symbols from port3, which has a loop length of 50 m. In order to account for the timedifferences between the ports, a CS may be employed.

In an embodiment, a Gigabit Passive Optical Network (GPON) standard mayutilize a scheme for upstream synchronization of optical signals, asdescribed in Recommendation ITU-T G. 984.3, entitled “Gigabit-capablePassive Optical Networks (G-PON): Transmission convergence layerspecification”, dated March 2008, which is incorporated herein byreference as if reproduced in its entirety. GPON's upstream alignmentmechanism may be similar to a method of time alignment and may employthe following steps. An optical line termination (OLT) unit may send anupstream request frame to an optical network termination (ONT) unit,with a time stamp recording the sending time T1. The ONT may receive theframe and record the time T2. After processing, the ONT may send an USregister frame and may record the sending time T3. The OLT may thencalculate the spread time (or propagation delay) of the loop as follows:

$\begin{matrix}{T_{spread} = \frac{T_{2} - T_{1} + T_{4} - T_{3}}{2}} & (1)\end{matrix}$

Next, the OLT may calculate the time that the ONT may delay after itreceives the upstream request frame by Delay=T_(max)−T_(spread). In anembodiment, T_(max) may be the spread time over a 60 km fiber. The OLTmay send the delay information to the ONT, which may use the delaybefore transmitting the US frames. After the ONT carries out the delay,the ONT's signal may appear to have been transmitted 60 km away from theOLT; thus, the US signal may be aligned to its assigned time-slot.Overall, this scheme may be highly complex, requiring severalcomplicated steps at the operator side. A simplified approach foralignment may be desirable. It is noted here that GPON in upstream usesTDMA time-division multiple access and in this system there should be notime-overlap among the upstream transmitted optical signals using thesame fiber as the communication media.

For the G.fast standard, a TDD method and a DMT method (e.g., orthogonalfrequency division multiplexing (OFDM)) may be chosen. In TDD, the USand DS may be divided in time rather than in frequency (as in FDD) toavoid local echo and NEXT from disturbing the received signal. The DSand US time slots may be synchronized and aligned among ports. In orderto reduce the effect of FEXT, all US symbols may be transmitted at thesame time similar to the method of timing advance used in FDD for VDSL2systems. However, the timing advance of FDD in VDSL2 systems may beoriginally proposed to counter echo and NEXT by forcing the CO and CPEtransmitters to start at the same time. Due to different loop lengths, aCS may be utilized to guarantee synchronization of all CPE transmittedsignals at the CO. In TDD methods, there may be no echo or NEXT sincethere is no time overlap between transmitter and receiver signals.Additionally, there may be no orthogonally issue of DS and US signals,as long as there is no time overlap between the two. Thus, the need forCS may be eliminated if all of the US signals from different linesarrive at a FTU-O (a G.fast transceiver unit at office) at the sametime.

Disclosed herein are systems, methods, and apparatuses for alignment ofUS DMT symbols of multiple lines in a TDD xDSL system. Schemes areproposed to achieve US symbol alignment at a transceiver unit at thecentral office side. Some embodiments may include estimating loop length(or equivalently propagation delay) at a transceiver unit at a remotecustomer side. Transceivers at the office side (e.g., FTU-O) may becollocated at a distribution point unit (DPU). If all of the US signalsfrom different lines use identical DMT symbol lengths with the equallengths of CP and arrive at the FTU-O at the same time, a CS may not benecessary. Thus, the upstream CS may be removed in a DMT symbolstructure, leading to less overhead and more efficient physical mediadependent (PMD) layer signaling.

Some DSL systems, such as G.fast, employ TDD for DS and US signaltransmission. G.fast modems may perform on loop lengths up to 400 mlong, along with a maximum frequency of 250 MHz for data transmission.In an embodiment, a short loop for G.fast may be 50 m or less. By way offurther example, suppose FTU-Rs transmit upstream signals at the sametime, and the corresponding FTU-O's are collocated. In this case, thedifference in arrival times of the upstream signals at FTU-O's may be aslarge as 2 microseconds (μs). This may indicate that if the CS is usedto cover the difference, the CS may need to be at least 2 μs long. Thisvalue may be a large overhead when the symbol period is in the range ofonly a few microseconds, which may be typical for G.fast. Eliminatingthe CS length may subsequently improve data throughput in a system.

In a DSL system such as a G.fast system, an FTU-R may receive downstreamsymbols and estimate or measure the signal propagation delay (sometimesreferred to as signal spread time) from FTU-O to FTU-R or from FTU-R toFTU-O. The propagation delay may be denoted as T_(pd) _(_) _(R). Inorder to implement an upstream symbol alignment method, a delay valueT_(max) may also be defined. FTU-R may calculate T2, which is the timethe receiver may need to wait before transmitting the US symbol. T2 maybe determined as follows:

T2=T _(max)−2·T _(pd) _(_) _(R)  (2)

T_(max) may represent an upper bound or maximum delay that a receivermay wait to transmit an US symbol. T_(max) may, for example, be twotimes the propagation delay of the longest loop as defined by an FTU-O.

After the downstream time slot, the FTU-R may wait for a time periodequivalent to the value of T2 plus the FTU-R switching time beforetransmitting the upstream symbol. The FTU-R may use the FTU-R switchingtime to prepare the upstream symbols after receiving the last symbol. Inan embodiment, the switching time may be zero. The switching time is thetime it takes for the transceiver to prepare to transmit the firstsample after receiving the last received signal sample.

If the FTU-R's estimation of the loop length is inaccurate, the upstreamsymbol alignment may be further enhanced as follows. FTU-R may sendspecial upstream symbols to FTU-O, and FTU-O may receive these symbolsand send a tuning information ΔT to FTU-R. This information may bereceived by FTU-R and may be used to modify and customize the sendingtime of the FTU-R's upstream symbols accordingly.

FIGS. 7A, 7B, and 7C illustrate embodiments of downstream symbolalignment at different time points. The system 700 comprises FTU-O-1702, FTU-O-2 704, FTU-R-1 712, FTU-R-2 714, FTU-O controller 718.FTU-O-1 702 and FTU-R-1 712 may be an office-side transceiver and acustomer-side transceiver, respectively, for port 1 in a G.fast system.Similarly, FTU-O-2 704 and FTU-R-2 714 may be an office-side transceiverand a customer-side transceiver, respectively, for port 2 in the samesystem. The U-interface may be a common interface for operator-sidetransceivers such as FTU-O-1 702 and FTU-O-2 704. The connection betweenan FTU-R and an FTU-O is through a copper pair, with the customerpremises endpoint designated as the U-R reference point (or U-interfacefor short) and the network endpoint designated as the U-0 referencepoint (or U-interface for short). Although only two FTU-O's 702 and 704and two FTU-Rs 712 and 714 are shown for illustrative purposes, anynumber of each component may be utilized for a plurality of ports in anxDSL system.

An FTU-O controller 718 may be a controller software or hardware entityutilized to coordinate and manage the operation of all FTU-O's. At time0, system 700 shows that DS symbol alignment between ports may easily beachieved by transmitting the DS symbols at the same time due tocollocation of all of the FTU-O's 702, 704. The transmitted DS symbolsfrom all of the ports may be aligned at FTU-O's to facilitate vectoringand FEXT cancellation.

System 700 depicts two ports with different loop lengths. Port 2 ofFTU-O-2 704 may comprise a shorter loop than port 1 of FTU-O-1 702.Thus, the short loop's FTU-R-2 714 may receive DS symbols earlier thanFTU-R-1 712. In FIG. 7B, system 700 at time 1 shows that the FTU-R-2 714may receive DS symbols earlier than FTU-R-1 712 due to the short loop inport 2. The FTU-R-2 714 may then estimate the propagation delay (T_(pd)_(_) _(R2)) from FTU-O-2 704 to FTU-R-2 714 and calculate the waitingtime T2 using the following equation:

T2=T _(max)−2·T _(pd) _(_) _(R2)  (3)

T_(max) in Equation 3 may be determined by FTU-O-2 and may be twice thepropagation delay of the longest loop. DS symbol may only reach FTU-R-1712 in the next time point, time 2, as depicted in FIG. 7C's snapshot ofthe symbol timing. Later on at port 1, the estimated delay time T_(pd)_(_) _(R1) may be greater than T_(pd) _(_) _(R2) determined by FTU-R-2714. Thus, FTU-R-1 712 may calculate the time that it may wait asfollows:

T2=T _(max)—2·T _(pd) _(_) _(R1)  (4)

The estimation of the propagation delay for a given loop may beimplemented in a number of ways, including but not limited to thefollowing algorithm: (1) estimate the loop attenuation from a receivedsignal (e.g., the received signal comprising at least one DMT symbol),(2) estimate the loop length from the loop attenuation, and (3) estimatethe loop delay time from the loop length. This algorithm may be employedin either an FTU-O (e.g., estimation of propagation delay based onreceived signal from an FTU-R) or an FTU-R (e.g., estimation ofpropagation delay based on received signal from an FTU-O).

FIGS. 8A, 8B, and 8C illustrate TDD upstream symbol alignment atdifferent time points. The system 700 at time 3 in FIG. 8A illustratesthat FTU-R-1 712, which has a longer loop than the other transceiver,may transmit US symbols earlier than FTU-R-2 714. At time 4 in FIG. 8B,the FTU-R-1 712's US signal may reach a distance away from FTU-O-1 702that may be approximately equal to FTU-R-2 714's distance away fromFTU-O-2 704. At this point, the FTU-R-2 714 may start transmitting USsymbols to FTU-O-2 704. This may be conducted to account for propagationdelay and loop length differences.

By way of further example, suppose the last DS sample in FIG. 7C istransmitted at T0 and may arrive at FTU-R-2 714 at T0+T_(pd) _(_) _(R2).The FTU-R-2 714 may then wait for T_(max)−2·T_(pd) _(_) _(R2) beforetransmitting the first sample US. The following Equations 5 and 6 maydefine the transmit time, T_(transmit).

T _(transmit) =T0+T _(pd) _(_) _(R2) +T _(max)−2·T _(pd) _(_) _(R2)  (5)

T _(transmit) =T0+T _(max) −T _(pd) _(_) _(R2)  (6)

The propagation delay from FTU-R-2 714 to FTU-O-2 704 may be T_(pd) _(_)_(R2), so that the arrival time may be T0+T_(max). FTU-R-1 712's firstsample may also arrive at FTU-O-1 702 at time T0+T_(max), so that the USsignals may be aligned at the DPU where all FTU-O's are collocated.

At time 5 in FIG. 8C, the US symbols of both ports may arrive at thesame time at their respective office-side transceivers. FTU-R-1 712'sand FTU-R-2 714's US symbol alignment at FTU-O-1 702 and FTU-O-2 704,respectively, may depend on the loop length estimation accuracy. FIG. 9illustrates an embodiment of symbol alignment for ports of varying looplengths. The embodiment may comprise a cyclic prefix (CP) 910 andsamples 2N 920 and may represent one DMT symbol. Since the US symbolsarrive at the corresponding FTU-O's at the same time, the CS may not beneeded. FIG. 9's TDD upstream alignment embodiment contrasts with theembodiment in FIG. 6, in which CS is required due to the lack of symbolalignment in the ports with varying loop lengths.

FIG. 10 illustrates loop attenuation versus loop length for differentloop types at 75 MHz. FIG. 10 illustrates attenuation data for looptypes AWG26, AWG24, PE04, PE05, and TP100. These loop types are wellunderstood by a person of ordinary skill in the art in DSL systems. Forexample, AWG26 and AWG24 are American Wire Gauge (AWG) 26 and 24 gauge,respectively. Each of the loop types represented in FIG. 10 may transmitat 75 MHz or at another frequency value of interest, as 75 MHz waschosen for illustrative purposes. Assuming that the loop type is knownor can be estimated with reasonable accuracy, the attenuation curves canbe used by a receiver, such as FTU-R, to estimate loop length based onpower of a received signal. That is, a signal with known power may betransmitted to an FTU-R, and the loop length may be estimated based onthe attenuation. If the loop length is known, the propagation delay canbe determined from the loop length (because the speed of signal travelis known, e.g., 2×10⁸ m/s).

There may be different ways of dealing with an unknown loop type. In onealternative, an average of the curves shown in FIG. 10 may be used togenerate a curve that is applied regardless of loop type. In anotheralternative FTUs may have an apriori knowledge of the loop type. In yetanother alternative, the loop type may be estimated. Based on theattenuation in FIG. 10, if the loop type is unknown and the averagecurve is used, the loop length estimation error may be as large as 15%.If the loop type is estimated incorrectly, the loop length estimationerror may be 30%. However, if the loop type estimation is accurate, theloop length estimation error may theoretically be zero.

To estimate loop length and propagation delay when strong FEXT signalsare present, a signal uncorrelated with the FEXT signals may be used.For example, by making the synchronization symbol (so-called syncsymbol) of each line derived from a different PRBS generator, eachreceiver will be able to detect its corresponding direct-channel signaland reject signals from the FEXT channels; therefore, estimating thepropagation delay of the direct channel. Alternatively, the sync symbolsmaybe transmitted sequentially by each FTU in a TDMA fashion so thatthere will be no time overlap among the transmitted signals to eliminateFEXT. Sync symbols are defined and used in all xDSL standards includingVDSL2/G.993.2 and and G.fast.

In the case that the FTU-R's estimation of the loop length isinaccurate, an additional technique may be performed. The FTU-R (e.g.,FTU-R-1 712 or FTU-R-2 714) may send special US symbols to FTU-O (e.g.,FTU-O-1 702 or FTU-O-2 704). The corresponding FTU-O may receive thesesymbols and use correlation or other methods to estimate an alignmentoffset between the FTU-R US symbol and a reference. The FTU-O may thensend a tuning offset information Δt to FTU-R, which may receive thisinformation and tune the transmit time of the US symbols accordingly(e.g., advance or delay transmissions by Δt).

Without these techniques for proper initial US symbol alignment, ajoining line may transmit its initial special US symbol at an arbitraryoffset time that may cause significant problems for US FEXT cancellersof other lines. If a line uses the timing advance method of VDSL2, itsUS symbol may be off by 1 μs on a 200 m line if the speed of electronson copper is assumed to be 2×10⁸ m/s. Using the TDD symbol alignmentmethod described herein, the maximum offset may be 15% of the estimatedlength, which translates to 0.15 μs on a 200 m line. In this case, theremay be a seven-fold improvement in accuracy. With a symbol period of 20us, a 0.15 us offset may create a worst case scenario of −42.5 dB ofaccumulated noise leakage from the new joining line to other lines. Thisleakage may not be cancelled out by the upstream FEXT cancellers.However, noise of 42.5 dB below the signal may not be expected to causesignificant problems in G.fast. Without the disclosed method, the noiselevel may be only 25.6 dB below signal level that may make othervectored upstream lines unstable.

For symbol alignment, G.fast standard may employ a specific symbolstructure for upstream and downstream alignment. FIG. 11 illustrates anembodiment of consecutive DMT symbols for the G.fast standard. Thesymbol may comprise an original 2N samples, a cyclic prefix of lengthL_(CP), and a cyclic suffix of length L_(CS). The cyclic suffixes ofconsecutive symbols may overlap by β windowed samples. The length of CSL_(CS) may be equal to β. In an embodiment, the CS samples with lengthL_(CS)=β may be completely overlapped with the first β samples of the CPof the next symbol to undergo the windowing operation. As compared withthe DMT samples for VDSL2 in FIG. 3, no additional CS samples may betransmitted. Therefore, the length of the transmitted symbols may besimplified to 2N+L_(CP), and the aforementioned TDD US symbol alignmentmay subsequently be performed.

FIG. 12 shows an embodiment of a TDD frame structure for G.fast. Theformat of a TDD frame may be presented in FIG. 12 with the followingnotations describing the TDD frame parameters. Values of T_(g1) andT_(g2) are the gap times at the U-interface of the FTU-O, while T_(g1′)and T_(g2′) are the gap times at the U-interface of the FTU-R. Both theFTU-O and FTU-R may transmit in respect to downstream and upstreamsymbol boundaries, respectively. In all cases, the sumT_(g1)+T_(g2)=T_(g1′)+T_(g2′) may be equal to the duration of one DMTsymbol. The value of T_(g1′) may not exceed 9 μs.

T_(pd) may denote a propagation delay of a signal from the FTU-O toFTU-R and vice versa. The variable T_(F) may define the frame period.The TDD frame period may be an integer multiple of DMT symbol periods.Therefore, one TDD frame may contain M_(ds) symbol periods dedicated fordownstream transmission, M_(us) symbol periods dedicated for upstreamtransmission, and a total gap time (T_(g1)+T_(g2)) equal to one symbolperiod. Hence, T_(F) and T_(symb), the symbol period, may be defined inthe following Equations 7 and 8, respectively:

$\begin{matrix}{T_{F} = {\left( {M_{ds} + M_{us} + 1} \right) \times T_{sym}}} & (7) \\{T_{symb} = {\frac{1}{f_{DMT}} = \frac{{2N} + L_{CP}}{2N \times \Delta \; f}}} & (8)\end{matrix}$

The downstream transmit symbol boundary may be aligned at the TDD frameboundary. The default value of T_(F) may be any number greater than orequal to three (i.e., at least one US symbol, one DS symbol, and onesymbol of gap time). Other values of T_(F) may be employed for furtherstudy. In an embodiment, all valid values of T_(F) may be equal to orless than 36 symbols. The frame parameters M_(ds) and M_(us) may be setat initialization, according to the corresponding management informationbase (MIB) parameters.

In order to enable initial US symbol alignment (synchronization) at theDPU, the length of the total gap time at both FTU-O and FTU-R may belimited to one DMT symbol, and the value of T_(g1′) may not exceed 9 us.This information, including FIG. 12's TDD frame structure, may providenecessary information for an implementation of US alignment.

As mentioned, in order for the transmitted US DMT symbols to arrive atthe same time at the DPU, the FTU-R located on a shortest loop may starttransmitting after a longest delay, while the FTU-R located on a longestloop may start transmitting after a shortest delay. Since the FTU-R'smay be from different vendors, the transceivers may each utilizedifferent values for the shortest delay (T_(g1′) _(_) _(min)), which mayresult in problems in US symbol synchronization. The shortest delay,which may be referred to as switch time or switching time, may be ahardware limitation. To mitigate this issue, the standard may eitherimpose a strict value on the switching time or make it a parameter thatFTU-O's will send to an FTU-R at the early stage of initialization. Thismay ensure that each transceiver will be using the same value. Thisvalue may be the upper bound on switching time, which may be denoted asST_(U). For example, ST(i)≦ST_(U) for all values of i, where ST(i) maybe the switching time of FTU-R(i).

FIG. 13 is an embodiment of an FTU-O module 1300 which may implementtiming alignment. The FTU-O module 1300 may also be referred to as aDPU. The FTU-O module 1300 may comprise N FTU-O transceivers 1302, atiming control entity (TCE) 1304, a PHY block 1306, a vectoring controlentity (VCE) 1308, and a management entity (ME) 1310.

The PHY block 1306 may represent the physical layer of the FTU-O module1300 towards the access network and of the network termination (NT)towards the customer premises (CP). The L2+ blocks represent the Layer 2and above functionalities contained in the FTU-O module 1300 and the NT.These blocks may be shown for completeness of the data flow. FIG. 13shows the reference model with the logical information flows within theFTU-O module 1300. The common element of all forms of coordination maybe synchronous and coordinated transmission or synchronous andcoordinated reception of signals from all N wire pairs connected to theFTU-O module 1300 (e.g., the coordinated group). Thus, the signals maybe represented as a vector where each component may be the signal on oneof the lines (shown as thick lines in FIG. 13). The management of anFTU-O module 1300 may be performed by the NMS, passing managementinformation to the ME 1310. Inside the FTU-O module 1300, the ME 1310conveys the management information (over an interface here called TCE-m)to the TCE 1304. The VCE 1308 coordinates the crosstalk cancellationover the coordinated group.

The TCE 1304 may be employed at the DPU in order to send the longestpossible loop length L_(max), or its corresponding propagation delayD_(max), and ST_(U) to the FTU-R's through FTU-O transceivers 1302. Thismay allow the FTU-R's to perform the aforementioned US symbolsynchronization disclosed herein.

Alternatively, TCE 1304 may send a T_(g2) value to FTU-R's. The FTU-R'smay then compute T_(g1′) as follows:

T _(g1)′(i)=T _(g2)−2×D(i)  (9)

D(i) may be the one-way propagation delay of FTU-R(i) connected toFTU-O(i). US symbol synchronization may be achieved if each FTU-Rfollows the above Equation 9. However, the value of T_(g2) may depend onthe delay D_(max) of the longest possible loop length L_(max) within theDPU and ST_(U). T_(g2) may then be computed by the following equation:

T _(g2) =ST _(U)+2×D _(max) +K  (10)

K may be a constant and may be set to zero. The ST_(U) value may eitherbe communicated to all the FTU-R's to comply with the value or it may bespecified in the standard to enable initial US symbol synchronization.The value of ST_(U) may have to be kept as small as possible to allow alarge D_(max) for a desired T_(g2). By way of further example, supposethere is a propagation delay of 0.5 μs per 100 m in a copper wire. Thevalue of T_(g2) may be assumed to be no greater than 10 μs to allow atleast 9 μs for T_(g1). If the value of ST_(U) is set to 8 us, the looplength difference between the shortest and the longest loops in the DPUgroup may only be 200 m. If the value of ST_(U) is set to 6 us, the looplength variation may be up to 400 m. If the aforementioned rule isdisregarded, the initial US symbol synchronization and fine tuning inlater stages may not be performed properly.

Alternatively, TCE 1304 may send a T_(g1′) value to each of the FTU-R's.The value for the ith FTU-R may be T_(g1′)(i). As discussed previously,the propagation delay D(i) may be estimated in FTU-O(i) based on areceived signal from FTU-R(i) using an algorithm discussed previously.The TCE 1304 or other part of the DPU may compute T_(g1′)(i) based onthe computed D(i) and then FTU-O(i) may send the value of T_(g1′)(i) toFTU-R(i), for all i=1, 2, . . . ,n.

FIG. 14 is a flowchart of an embodiment of an upstream symbol alignmentmethod 1400. The method may be performed in a DPU. The method begins inblock 1410 in which a delay value may be transmitted to a plurality ofDSL transceivers (e.g., by corresponding transceivers or FTU-Os in theDPU). The delay value may be transmitted by representing the delay valueas a series of bits in a packet. The plurality of DSL transceivers maybe a plurality of CPEs. The delay value may be as described previously.For example, the delay value may be T_(max) or T_(g2) or both, variablesdescribed previously. In response, in block 1420 a plurality of signalsmay be received at substantially the same time. The signals may bereceived within some small tolerance of each other such that they appearfrom the perspective of the DPU to have effectively arrived at the sametime. In other words, any variation in arrival time of the plurality ofsignals has no detrimental effect on FEXT in the DPU. The method 1400may optionally contain the step of transmitting an upper bound (ST_(U))on switching time to the plurality of DSL transceivers.

The plurality of signals may be received at substantially the same timebecause each of the plurality of DSL transceivers transmitted itscorresponding signal at a different time as compared to the other DSLtransceivers to account for, e.g., differences in propagation delayand/or switching times. The plurality of DSL transceivers mayindividualize their transmit times according to, e.g., equations(2)-(4), (9), or (10).

FIG. 15 is a schematic diagram of an embodiment of a DPU 1500 configuredto perform at least one of the schemes described herein. The DPU 1500comprises a processor 1510, a memory device 1520, and a plurality oftransceivers 1530 configured as shown in FIG. 15 (there may be ntransceivers, where n is an integer greater than one). A U-interface isillustrated in FIG. 15 as an interface common to the transceivers 1530.The processor 1510 may be implemented as one or more central processingunit (CPU) chips, cores (e.g., a multi-core processor),field-programmable gate arrays (FPGAs), application specific integratedcircuits (ASICs), and/or digital signal processors (DSPs). The processor1510 may be implemented using hardware or a combination of hardware andsoftware.

The memory device 1520 may comprise a cache, random access memory (RAM),read-only memory (ROM), secondary storage, or any combination thereof.Secondary storage typically comprises one or more disk drives or tapedrives and is used for non-volatile storage of data and as an over-flowdata storage device if RAM is not large enough to hold all working data.Secondary storage may be used to store programs that are loaded into RAMwhen such programs are selected for execution. ROM may be used to storeinstructions and perhaps data that are read during program execution.ROM a non-volatile memory device that typically has a small memorycapacity relative to the larger memory capacity of secondary storage.RAM may be used to store volatile data and perhaps to storeinstructions. Access to both ROM and RAM is typically faster than tosecondary storage.

The transceivers 1530 may be FTU-Os and may be configured to perform DMTmodulation and demodulation. Each of the transceivers 1530 may becoupled to a corresponding CPE via a DSL line. The transceivers 1530 mayserve as input and/or output devices of the DPU. For example, if atransceiver 1530 is acting as a transmitter, it may transmit data out ofthe DPU 1500. If a transceiver 1530 is acting as a receiver, it mayreceive data into the DPU 1500.

The DPU 1500 may be configured to perform any of the schemes discussedherein, such as the method 1400. For example, the transceivers 1530 maytransmit a delay value to a corresponding CPE according to block 1410,as directed by the processor 1510. The transceivers 1530 may thereafterreceive a plurality of signals at substantially the same time (e.g., atthe U-interface) according to block 1420. The DPU 1500 may implement theFTU-O module 1300. For example, the TCE 1304 may be implemented in theprocessor 1510 and/or the memory 1520, and the FTU-O transceivers 1302may correspond to the transceivers 1530.

Note that a CPE may generally have the same configuration as the DPU1500 except that a CPE may have only one transceiver. That is, a CPE mayhave a memory, a processor, and a transceiver configured as shown inFIG. 15. The U-interface for a CPE may be a U-R interface.

It is understood that by programming and/or loading executableinstructions onto the DPU 1500, at least one of the processor 1510 andthe memory 1520 are changed, transforming the DPU 1500 in part into aparticular machine or apparatus (e.g., a DPU having the functionalitytaught by the present disclosure). The executable instructions may bestored on the memory 1520 and loaded into the processor 1510 forexecution. It is fundamental to the electrical engineering and softwareengineering arts that functionality that can be implemented by loadingexecutable software into a computer can be converted to a hardwareimplementation by well-known design rules. Decisions betweenimplementing a concept in software versus hardware typically hinge onconsiderations of stability of the design and numbers of units to beproduced rather than any issues involved in translating from thesoftware domain to the hardware domain. Generally, a design that isstill subject to frequent change may be preferred to be implemented insoftware, because re-spinning a hardware implementation is moreexpensive than re-spinning a software design. Generally, a design thatis stable that will be produced in large volume may be preferred to beimplemented in hardware, for example in an application specificintegrated circuit (ASIC), because for large production runs thehardware implementation may be less expensive than the softwareimplementation. Often a design may be developed and tested in a softwareform and later transformed, by well-known design rules, to an equivalenthardware implementation in an application specific integrated circuitthat hardwires the instructions of the software. In the same manner as amachine controlled by a new ASIC is a particular machine or apparatus,likewise a computer that has been programmed and/or loaded withexecutable instructions may be viewed as a particular machine orapparatus.

At least one embodiment is disclosed and variations, combinations,and/or modifications of the embodiment(s) and/or features of theembodiment(s) made by a person having ordinary skill in the art arewithin the scope of the disclosure. Alternative embodiments that resultfrom combining, integrating, and/or omitting features of theembodiment(s) are also within the scope of the disclosure. Wherenumerical ranges or limitations are expressly stated, such expressranges or limitations may be understood to include iterative ranges orlimitations of like magnitude falling within the expressly stated rangesor limitations (e.g., from about 1 to about 10 includes, 2, 3, 4, etc.;greater than 0.10 includes 0.11, 0.12, 0.13, etc.). For example,whenever a numerical range with a lower limit, R₁, and an upper limit,R_(u), is disclosed, any number falling within the range is specificallydisclosed. In particular, the following numbers within the range arespecifically disclosed: R=R₁+k*(R_(u)−R₁), wherein k is a variableranging from 1 percent to 100 percent with a 1 percent increment, i.e.,k is 1 percent, 2 percent, 3 percent, 4 percent, 5 percent, . . . , 50percent, 51 percent, 52 percent, . . . , 95 percent, 96 percent, 97percent, 98 percent, 99 percent, or 100 percent. Moreover, any numericalrange defined by two R numbers as defined in the above is alsospecifically disclosed. The use of the term “about” means+/−10% of thesubsequent number, unless otherwise stated. Use of the term “optionally”with respect to any element of a claim means that the element isrequired, or alternatively, the element is not required, bothalternatives being within the scope of the claim. Use of broader termssuch as comprises, includes, and having may be understood to providesupport for narrower terms such as consisting of, consisting essentiallyof, and comprised substantially of. Accordingly, the scope of protectionis not limited by the description set out above but is defined by theclaims that follow, that scope including all equivalents of the subjectmatter of the claims. Each and every claim is incorporated as furtherdisclosure into the specification and the claims are embodiment(s) ofthe present disclosure. The discussion of a reference in the disclosureis not an admission that it is prior art, especially any reference thathas a publication date after the priority date of this application. Thedisclosure of all patents, patent applications, and publications citedin the disclosure are hereby incorporated by reference, to the extentthat they provide exemplary, procedural, or other details supplementaryto the disclosure.

While several embodiments have been provided in the present disclosure,it may be understood that the disclosed systems and methods might beembodied in many other specific forms without departing from the spiritor scope of the present disclosure. The present examples are to beconsidered as illustrative and not restrictive, and the intention is notto be limited to the details given herein. For example, the variouselements or components may be combined or integrated in another systemor certain features may be omitted, or not implemented.

In addition, techniques, systems, subsystems, and methods described andillustrated in the various embodiments as discrete or separate may becombined or integrated with other systems, modules, techniques, ormethods without departing from the scope of the present disclosure.Other items shown or discussed as coupled or directly coupled orcommunicating with each other may be indirectly coupled or communicatingthrough some interface, device, or intermediate component whetherelectrically, mechanically, or otherwise. Other examples of changes,substitutions, and alterations are ascertainable by one skilled in theart and may be made without departing from the spirit and scopedisclosed herein.

What is claimed is:
 1. A method for time-division duplex (TDD)transmission in a digital subscriber line (DSL) communication systemcomprising a group of DSL transceivers at a remote side (FTU-Rs) and adistribution point unit (DPU), comprising: performing, with a secondFTU-R among the group of FTU-Rs, an initial upstream symbol alignmentfor a first FTU-R, wherein a first gap time that the first FTU-R needsto wait before transmitting first upstream symbols to a first DSLtransceiver at an operator side (FTU-O) at the DPU in a TDD frame periodis acquired to make the first upstream symbols align at the firstFTU-O's interface; transmitting second upstream symbols to the firstFTU-O to correct the first gap time after performing the initialupstream symbol alignment; receiving, from the first FTU-O, tuningoffset information representing a correction for the first gap time; andtuning the first FTU-R's upstream symbols transmit time according to thetuning offset information received.
 2. The method of claim 1, whereinthe first gap time is calculated based on an estimation of loop length.3. The method of claim 2, wherein the first gap time is estimated by thefirst FTU-R.
 4. The method of claim 3, further comprising: receiving,from the first FTU-O, one or more parameters of a longest possibleloop-length, propagation delay, or upper bound on switching time;applying the one or more parameters to estimate the first gap time. 5.The method of claim 1, wherein the first gap time corresponds to alongest loop for the DPU.
 6. An apparatus for a time-division duplex(TDD) transmission in a digital subscriber line (DSL) transceiver atremote side (FTU-R), comprising: a memory storage comprisinginstructions; and one or more processors coupled to the memory thatexecute the instructions to: enable the FTU-R to perform an initialupstream symbol alignment with a second FTU-R that supports TDDtransmission with a first gap time that the FTU-R needs to wait beforetransmitting first upstream symbols to a first DSL transceiver at anoperator side (FTU-O) in a TDD frame period acquired to make the firstupstream symbols align at the first FTU-O's interface; transmit secondupstream symbols to the first FTU-O to correct the first gap time afterperforming the initial upstream symbol alignment; receive, from thefirst FTU-O, tuning offset information that represents a correction forthe first gap time; and tune the FTU-R's upstream symbols transmit timeaccording to the tuning offset information.
 7. The apparatus of claim 6,wherein the first gap time is estimated based on an estimation of looplength.
 8. The apparatus of claim 7, wherein the one or more processorsexecute the instructions to estimate the first gap time.
 9. Theapparatus of claim 8, wherein the one or more processors execute theinstructions to: receive, from the first FTU-O, one or more parametersof a longest possible loop-length, propagation delay, or upper bound onswitching time; apply the one or more parameters to estimate the firstgap time.
 10. The apparatus of claim 6, wherein the first gap timecorresponds to a longest loop for the DPU.
 11. A distribution point unit(DPU) comprising: a plurality of digital subscriber line (DSL)transceivers that support time-division duplex (TDD) transmissions withrespective DSL transceivers at remote side (FTU-Rs); and a processorcoupled to the DSL transceivers and configured to: perform an initialupstream symbol alignment for a first FTU-R coupled to a first DSLtransceiver among the plurality of DSL transceivers with a second FTU-Rcoupled to a second DSL transceiver among the plurality of DSLtransceivers, wherein a first gap time that the first FTU-R needs towait before transmitting first upstream symbols to the first DSLtransceiver in a TDD frame period is acquired to make the first upstreamsymbols align at the first DSL transceiver's interface; estimate acorrection for the first gap time to be used by the first FTU-R to tuneupstream symbols transmit time in response to reception of secondupstream symbols for correcting the first gap time from the first FTU-R;and transmit, to the first FTU-R, tuning offset information thatrepresents a correction for the first gap time.
 12. The DPU of claim 11,wherein the first gap time is estimated based on an estimation of looplength.
 13. The DPU of claim 11, wherein the processor is configured toexecute instructions to transmit one or more parameters of a longestpossible loop-length, propagation delay, or upper bound on switchingtime to the first FTU-R to estimate the first gap time.
 14. The DPU ofclaim 11, wherein the first gap time corresponds to a longest loop forthe DPU.